The present invention pertains to an improved process for the packaging of hot-melt, pressure-sensitive adhesives in the form of blocks with surfaces that are neither tacky nor sticky.
It can be briefly summarized that pressure-sensitive adhesives are substances that can adhere to various supports and to themselves upon simple contact under a pressure of varying strength. They are substances or mixtures of substances endowed with specific viscoelastic properties, which are thoroughly discussed by S. G. Chu in Viscoelastic Properties of Pressure-Sensitive Adhesives, Handbook of Pressure-Sensitive Adhesive Technology, Van Nostrand Reinhold Co. They are most frequently compositions based on polymers or elastomers, e.g., natural rubber, SBR, SIS, SBS rubbers, polychloroprene, nitrile rubber, chlorinated rubber, butyl rubbers, ethylene-vinly acetate copolymers, polyertheramides, polymers or copolymers of alpha-olefins, and various ingredients that are well known by the expert in the field, which include tackifying resins, waxes, plasticizers and other ordinary formulation ingredients.
These compositions are valued for their dual specific characteristics of exhibiting a strong, instantaneous adhesive power in the melted state (referred to as tack) and of preserving a surface adhesive power upon their return to the solidified state. This latter behaviour is obviously disadvantageous for the handling and storage of blocks, cakes, slugs and other individual forms of packaging these pressure-sensitive adhesives.
Several procedures are known for resolving this drawback. First of all, there are the widespread solutions that are comprised of using removable, disposable packaging for blocks of pressure-sensitive adhesives with foils or films made of a material on which the adhesive property of the pressure-sensitive adhesive does not develop; such as packaging of individual preformed blocks with foils, pouring the melted pressure-sensitive adhesive into boxes, and pouring the pressure-sensitive adhesive into resuable molds that have first been covered with a film made of such a material. They all have all of the drawbacks of disposable packaging systems, and the drawback of handling blocks of pressure-sensitive adhesives that after unpacking retain the drawback of blocks with a sticky surface.
Other procedures employ a protective layer made of a material that upon use undergoes the same fate as the pressure-sensitive adhesives, the most valuable being the procedures in which the pressure-sensitive adhesive is poured into a mold that has previously been primed with such a material. Thus, there is known the procedure of pouring the pressure-sensitive adhesive into molds that have previously been powdered, the powdered envelope of which is obtained and maintained in a coherent state by electrostatic means, described in the French patent No. 2,541,930. There is also known the procedure of pouring the pressure-sensitive adhesive into a mold covered with a web of a hot-melt but not pressure-sensitive adhesive described in the French patent application No. 2,601,616. The first procedure has the problems associated with implementation of an electrostatic procedure which, in addition, does not act to protect the casting surface. The second procedure has the problems of eliminating the burrs on the finished blocks of pressure-sensitive adhesives and the fouling of the machines by the filaments that escape during formation of the hot-melt adhesive web.